Even though there has been considerable study of alternative electrochemical systems, the lead-acid battery is still the battery of choice for general purposes such as starting a vehicle, boat or airplane engine, emergency lighting, electric vehicle motive power, energy buffer storage for solar-electric energy, and field hardware, both industrial and military. These batteries may be periodically charged from a generator or other source of suitable DC power.
The conventional lead-acid battery is a multi-cell structure. Each cell generally comprises a set of vertical interdigitated monopolar positive and negative plates formed of lead or lead-alloy grids containing layers of electrochemically active pastes or active materials. The paste on the positive electrode plate when charged comprises lead dioxide (PbO.sub.2), which is the positive active material, and the negative plate contains a negative active material such as sponge lead. An acid electrolyte based on sulfuric acid is interposed between the positive and negative plates.
Lead-acid batteries are inherently heavy due to the use of the heavy metal lead in constructing the plates. Modern attempts to produce light-weight lead-acid batteries, especially in the aircraft, electric car and vehicle fields, have placed their emphasis on producing thinner plates from lighter weight materials used in place of and in combination with lead. The thinner plates allow for the use of more plates for a given volume, thus increasing the power density of a conventional lead-acid battery. However, the extent to which conventional battery performance can be improved upon is limited by its inherent construction.
Bipolar batteries are not new and have been known for some time and offer the potential for improvement over monopolar battery technology. Bipolar battery construction comprises a series of electrode plates that each contain a negative active material on one side and a positive active material on the other side, hence the terms "bipolar" and "biplate". The biplates are serially arranged in such a fashion that the positive side of one plate is directed toward the negative side of an opposing plate. The bipolar battery is made up of separate electrolytic cells that are defined by biplates of opposing polarities. The biplates must be impervious to electrolyte and be electrically conductive to provide a serial connection between cells.
The bipolar battery is characterized by having improved current flow over that of conventional monopolar batteries. The enhanced current flow is the result of through-plate current transfer from one polarity of the biplate to the other. In a conventional monopolar battery the current must travel from one electrode plate to another of opposite polarity via a conductive path which commonly is circuitous and of relatively considerable length. The significantly shortened intercell current path inherent in the bipolar battery reduces the battery's internal resistance, making it more efficient than the conventional monopolar battery in both discharging and charging modes of operation. This reduced internal resistance permits the construction of a bipolar battery that is both smaller and lighter than its equivalent monopolar battery, making it a highly desirable alternative for use in the aircraft, military and electric vehicle industry where considerations of size and weight are of major importance.
The bipolar battery, however, is not without its own difficulties and problems which, heretofore, have resisted efficient resolution and solution. A first such difficulty is related to the choice of materials for the conductive sheet used to make up the biplate. The bipolar construction in its simplest form would use lead or lead alloy for the conductive sheet and intercell partition. However, since the lead of the conductive sheet is corroded at the positive (anode) side by both the action of overcharge (recharge) and by the current-producing interaction between the conductive sheet and the active material itself (discharge), in time the lead sheet will be corroded entirely through from the positive side. Once penetrated, the electrolyte from the cell on the positive side has direct contact with the electrolyte on the negative side and a short circuit is established between adjacent cells. In short order, the positive material on one side of the now-penetrated conductive sheet becomes fully discharged against the negative active material on the opposite side.
This short circuit condition will result not only in a loss of voltage from the two cells, but will also introduce a very high resistance in the series-connected bipolar cells due to the near total conversion of both active materials to lead sulfate, which is nonconducting. The greatly increased resistance will render the battery nearly useless at all but very low discharge rates and will entirely defeat the fundamentally low resistance inherent in bipolar construction.
Another problem in the construction of the bipolar battery is one inherent with the construction of lead-acid batteries in general. This involves achieving an electrolyte-tight seal about a conductive battery member passing from a position inside the battery and in intimate contact with the battery electrolyte through the battery housing to the battery's outside surface. In a conventional monopolar lead-acid battery, problems of unwanted electrolyte leakage occurs at the terminal post seal where a terminal post (conductive member) passes through the battery cell cover. The unique electro-chemistry occurring at such seal interface predicts that a true seal cannot be obtained so long as an oxide film exists on the conductive member. Eventually the seal will fail either by creep corrosion, resulting in the migration of the electrolyte to the battery surface, or around the end of the conductive member to the adjacent cell, or fail by crevice corrosion (also called nodular corrosion) resulting in the mechanical separation or failure of the seal.
The problem related to achieving an electrolyte-tight seal about a conductive battery member in intimate contact with the battery electrolyte as the conductive member passes through the battery housing is well known and understood in the art and was addressed in a paper entitled "Vulcanized Rubber Post Seal For Lead-Acid Batteries A New Generic Type" presented before the 1988 INTELEC International Telecommunications Energy Conference. The paper identified two different types of corrosion mechanisms, creep and crevice or nodular corrosion, that was responsible for electrolyte migration in electrolytic batteries.
In bipolar battery construction, a bipolar plate is the conductive member which is in intimate contact with the battery electrolyte. The bipolar plate extends from its position in contact with the electrolyte to a position at its edges where it is sealed to a barrier between adjacent cells; it may extend to an exterior surface of the battery. At the position defining the battery surface the biplate must interact with other battery components to form an electrolyte-tight seal. The problem of achieving an electrolyte-tight seal is exacerbated in bipolar construction because the electrolyte-tight seal must occur about the full perimeter of each biplate, making the total area of seal much greater than that encountered in conventional monopolar battery construction.
It is seen, therefore, that a need exist for a bipolar battery electrode plate construction that will afford improved protection from chemical and electrolytic attack, serving to both extend battery service life and improve the predictability of battery performance. It is desirable that the electrode plate be constructed using methods commercially and economically feasible from materials readily and practically available. It is also desirable that the electrode plate construction facilitate the reclamation and recovery of recyclable materials used in battery construction. Further, it is highly desirable that the battery electrode plate be constructed in a manner facilitating an electrolyte-tight seal when used in conjunction with other battery members.